Write setup and teardown once

Port of Python's contextlib.contextmanager. R has no yield, so instead of suspending the function the body calls teardown() right after acquiring each resource and returns the resource. From that one definition you get:

Usage

contextmanager(f)

teardown(expr)

Arguments

expr

Code that releases the resource. Evaluated in the setup function's environment when the context ends, so it can use the setup's local variables.

f

The function to decorate. Usually supplied by %@% rather than directly.

Value

contextmanager is a decorator. teardown() is called for its side effect.

Details

• a local_ form: the function itself. Cleanup runs when the calling function exits, like withr::local_*().

• a with_ form: with_context() runs a block, then cleans up.

• a decorator: as_decorator() runs a function inside the context.

The decorated function takes ..., passed on to the setup function, and a .local_envir argument naming the frame whose exit triggers cleanup. It defaults to the caller, or, when called from another context manager's setup, to that context's .local_envir, so composed contexts last as long as the outer one.

teardown(expr) registers expr with withr::defer() on that frame. Cleanups run last in, first out, and the ones already registered still run if the rest of the setup fails. teardown() finds the frame by searching the call stack for the nearest running context manager and reading its .local_envir.

See also

with_context(), as_decorator()

Examples

local_tempdir := contextmanager %@% function(pattern = "tmp") {
  path <- tempfile(pattern)
  dir.create(path)
  teardown(unlink(path, recursive = TRUE))
  path
}

f <- function() {
  d <- local_tempdir()
  writeLines("hi", file.path(d, "x.txt"))
  d
}
d <- f()
dir.exists(d) # cleaned up when f() exited
#> [1] FALSE